Measurement of the Beam-Spin Azimuthal Asymmetry Associated with Deeply-Virtual Compton Scattering

Abstract

The beam-spin asymmetry in hard electroproduction of photons has been measured for the first time. The data have been accumulated by the HERMES experiment at DESY using the HERA 27.6 GeV longitudinally polarized positron beam and an unpolarized hydrogen gas target. The asymmetry in the azimuthal distribution of the produced photons in the angle phi relative to the lepton scattering plane was determined with respect to the helicity state of the incoming positron beam. The beam-spin analyzing power in the sin(phi) moment was measured to be -0.23 +/- 0.04(stat) +/- 0.03(syst) in the missing-mass range below 1.7 GeV. The observed asymmetry is attributed to the interference of the Bethe-Heitler and deeply-virtual Compton scattering processes.

Figures (5)

• Figure 1: (a) Feynman diagram for deeply-virtual Compton scattering, and (b) photon radiation from the incident and scattered lepton in the Bethe-Heitler process.
• Figure 2: The measured distribution of photons observed in hard electroproduction versus the missing mass squared ${M_x}^2$. In the upper panel the full kinematic range is displayed, while the low ${M_x}^2$ domain is shown in the lower panel. The light-gray histogram represents the results of a Monte-Carlo simulation in which fragmentation processes and the Bethe-Heitler process are included, while the dark-shaded histogram represents only the BH contribution. The Monte-Carlo simulation includes the effect of the detector resolution.
• Figure 3: Beam-spin asymmetry $A_{LU}$ for hard electroproduction of photons as a function of the azimuthal angle $\phi$. The data correspond to the missing mass region between -1.5 and +1.7 GeV. The dashed curve represents a $\sin\phi$ dependence with an amplitude of 0.23, while the solid curve represents the result of a model calculation taken from Ref. [17]. The horizontal error bars represent the bin width, and the error band below represents the systematic uncertainty.
• Figure 4: The $\sin\phi$-moment $A_{LU}^{\sin\phi^{\pm}}$ as a function of the missing mass for positive beam helicity (circles), negative beam helicity (squares) and the averaged helicity (open triangles). A negative value is assigned to $M_x$ if $M_x^2 <$ 0. The error bars are statistical only. The systematic uncertainty is represented by the error band at the bottom of the figure.
• Figure 5: The beam-spin analyzing power $A_{LU}^{\sin\phi}$ for hard electroproduction of photons on hydrogen as a function of the missing mass. The systematic uncertainty is represented by the error band at the bottom of the figure.